1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming a multi-color image by superposing in succession color component images formed according to the image information of respective color components, and a controlling method for such image forming apparatus.
2. Related Background Art
For printing color image data, there is conventionally known a color image forming apparatus, such as a laser beam printer (LBP), adapted to form a color image by forming a latent image, line by line, on a photosensitive member utilizing main scanning means such as a rotary polygonal mirror which scans the photosensitive member with a laser beam, then forming an image of each color component by developing the latent image with the developer of each color component such as magenta (M), cyan (C), yellow (Y) or black (Bk), and transferring the images of respective color components in superposed manner onto a sheet fixed on a transfer drum.
There is also known an apparatus in which the images of respective color components, formed in succession on the photosensitive member, are transferred in superposed manner on an intermediate transfer member and the color images on the intermediate transfer member are collectively transferred onto the sheet.
In such apparatus, the photosensitive member and the transfer drum or the intermediate transfer member are driven at a constant speed in a direction (sub scanning direction) perpendicular to the main scanning direction, and the superposed image transfer, color by color, onto the sheet on the transfer drum or onto the intermediate transfer member is conducted in synchronization with a sub scanning start signal generated at each rotation of the photosensitive drum, the transfer drum or the intermediate transfer member.
There is also known an apparatus capable of forming images of respective color components in superposed manner on the photosensitive member and collectively transferring such images onto the recording sheet.
In such conventional color image forming apparatus as described in the foregoing, in order to prevent deterioration in the image quality of the color image resulting from the aberration in the superposing position of the color images, the method of controlling the positions of the respective color images in superposition plays an important role.
As an example of such position controlling method, there is proposed a method of employing such a configuration that the number of the sub scan start signal (ITOP signal) generated during a rotation of the photosensitive member or the intermediate transfer member and that of the main scan recording line signal (BD signal) synchronized therewith become an integer (cf. FIG. 9B) thereby synchronizing the rotation of the motor driving the photosensitive member or the intermediate transfer member and that of the scanner motor for effecting the main scanning.
FIGS. 9A and 9B are schematic views showing main scanning lines formed on the photosensitive member or the intermediate transfer member in the conventional image forming apparatus.
Referring to FIGS. 9A and 9B, there is shown an image bearing member 901 such as a photosensitive member or an intermediate transfer member, which will be represented by a photosensitive member in the following description. An ITOP sensor 802 generates a sub scanning start signal (ITOP signal) by detecting a sensor flag 803 provided in a predetermined position on the lateral face of the photosensitive member 801 at each rotation thereof.
FIG. 9A shows a case where the number of the main scanning start signal (BD signal) obtained during a rotation of the photosensitive member 801 and that of the main scan recording line signals synchronized therewith becomes "n+(1/2)" (n being an integer), and illustrates the positions of the main scan recording line signal in the first, second, . . . , (n-1)th, n-th lines of a first rotation and in the first line in a second rotation.
As shown in FIG. 9A, during a rotation of the photosensitive member 801, namely during the generation of the ITOP signal, the main scan recording line signal is generated by "n+(1/2)", so that the first line in the first rotation and the first line in the second rotation are mutually displaced by 1/2 line corresponding to the fraction.
In the conventional image forming apparatus, in order to avoid such displacement between the first and second rotations, there is proposed such a configuration that the number of the main scan start signal (BD signal) generated during a rotation of the aforementioned photosensitive member or intermediate transfer member and that of the main scan recording line signal synchronized therewith become an integer, as exemplified in FIG. 9B.
FIG. 9B shows a case where the number of the main scanning start signal (BD signal) obtained during a rotation of the photosensitive member 801 and that of the main scan recording line signals synchronized therewith become n (n being an integer), and illustrates the positions of the main scan recording line signal in the first, second, . . . , (n-1)th, n-th lines of a first rotation and in the first line in a second rotation.
As shown in FIG. 9B, during a rotation of the photosensitive member 801, namely during generation of the ITOP signal, the main scan recording signal is generated by n (integer), so that the first line in the first rotation and the first line in the second rotation mutually overlap without aberration.
In the following there will be explained, with reference to FIGS. 10 and 11, a method of synchronizing the rotation of the motor for driving the photosensitive member or the intermediate transfer member and that of the scanner motor for effecting the main scanning, in the conventional image forming apparatus.
A first method consists of dividing the frequency of the main scanning start signal (BD signal), generated along the rotation of the scanner motor, and utilizing thus obtained signal as the reference clock signal for the motor for driving the photosensitive member or the intermediate transfer member. In the following there will be explained an example of such configuration.
FIG. 10 shows the configuration of a conventional image forming apparatus, corresponding to the first method described above.
Referring to FIG. 10, a photosensitive member 901 is rotated, through a driving belt 908, by a photosensitive member driving motor 907. A scanner motor 902 is controlled at a constant revolution by a PLL circuit 910, based on a reference clock signal supplied from an oscillator 911, and rotates a polygonal mirror 903. The polygonal mirror 903 deflects a laser beam emitted by a laser 904, thereby line scanning the surface of the photosensitive member 901.
A beam detecting sensor (BD sensor) 906 is positioned in a non-image area on the scanning line of the laser beam, and generates a main scanning start signal (BD signal) in each scanning line of the laser beam, namely in synchronization with the rotation of the scanner motor. A PLL circuit 909 effects constant-speed control of the photosensitive member driving motor 907, utilizing the BD signal, generated by the BD sensor 906, as the reference clock signal. In this manner the rotation of the scanner motor 902 and that of the photosensitive member driving motor 907 can be synchronized.
A second method consists of utilizing a common clock signal as the reference clock signal for the motor for driving the photosensitive member or the intermediate transfer member and that for the scanner motor effecting the main scanning. In the following there will be explained an example of such configuration.
FIG. 11 shows the configuration of a conventional image forming apparatus, corresponding to the second method described above.
Referring to FIG. 11, a photosensitive member 1001 is rotated, through a driving belt 1008, by a photosensitive member driving motor 1007. A scanner motor 1002 is controlled at a constant revolution by a PLL circuit 1010, based on a reference clock signal supplied from an oscillator 1011, and rotates a polygonal mirror 1003. The polygonal mirror 1003 deflects a laser beam emitted by a laser 1004, thereby line scanning the surface of the photosensitive member 1001.
A PLL circuit 1009 effects constant-speed control of the photosensitive member driving motor 1007, utilizing a reference clock signal generated by an oscillator 1011, used for the PLL control of the scanner motor 1002. In this manner the rotation of the scanner motor 1002 and that of the photosensitive member driving motor 1007 can be synchronized.
By synchronizing the rotation of the motor driving the photosensitive member and that of the scanner motor for controlling the main scanning by the aforementioned first or second method while adopting such a configuration that the number of the main scanning start signal (BD signal) generation during a rotation of the photosensitive member or the intermediate transfer member and that of the main scanning recording line signal become an integer, it is rendered possible to achieve position alignment without aberration in the start position of sub scanning, even after a number of rotations of the photosensitive member or the intermediate transfer member.
For controlling the sub scanning start position, there is also known a third method for matching the phase of the main scanning start signal and the sub scanning start signal, enabling position alignment regardless whether the number of the main scanning start signal (BD signal) obtained during a rotation of the photosensitive member or the intermediate transfer member and that of the main scanning recording line signal synchronized therewith is an integer or not. In the following there will be explained an example of such configuration.
FIG. 12 shows the configuration of a conventional image forming apparatus, corresponding to the above-described third method.
Referring to FIG. 12, a photosensitive member 1101 is rotated, through a driving belt 1108, by a photosensitive member driving motor 1107. A PLL circuit 1109 effects constant-speed control of the photosensitive member driving motor 1107, utilizing a reference clock signal generated by an oscillator 1114. An ITOP sensor 1115 generates an ITOP signal when the ITOP sensor 1115 is shielded by a sensor flag 1116 in each rotation of the photosensitive member 1101. The writing start position of the first line on the surface of the photosensitive member 1101 is determined, based on the ITOP signal.
A phase matching circuit 1112 effects phase matching between the reference clock signal generated by an oscillator 1113 and the ITOP signal generated by the ITOP sensor 1115. A PLL circuit 1110 effects constant-speed control of the scanner motor 1102 based on the reference clock signal which is phase matched with the ITOP signal by the phase matching circuit 1112.
By the phase matching of the ITOP signal and the reference clock signal by the phase matching circuit 1112, the rotational phase of the scanner motor 1102 is corrected always at a same value at each ITOP signal. Consequently the rotational phase of the polygonal mirror 1103 driven by the scanner motor 1102 is synchronized with the ITOP signal, and the line scanning position of the laser beam, coming from the laser 1104 through the lens 1105, on the surface of the photosensitive member 1101 is always maintained same with reference to the ITOP signal.
FIG. 13 is a schematic view showing the relationship between the actual main scanning lines (main scanning start signal) and the ITOP signal (sub scanning start signal) on the photosensitive member of a conventional image forming apparatus.
Referring to FIG. 13, an image bearing member 1301, such as a photosensitive member or an intermediate transfer member, will be explained hereinafter as a photosensitive member. An ITOP sensor 1302 generates a sub scanning start signal (ITOP signal) by detecting a sensor flag 1303 provided in a predetermined position on the lateral face of the photosensitive member 1301, in each rotation thereof.
A rotation of the photosensitive member 1301 consists of "n+(1/2)" lines (n being an integer). The ITOP sensor 1302 generates the sub scanning start signal at a predetermined position in each rotation of the photosensitive member 1301. In such configuration, since "n+(1/2)" main scanning lines are generated during a rotation of the photosensitive drum, the first line in the first rotation and the first line in the second rotation are displaced by the fraction of 1/2 lines as shown in FIG. 9A.
It is however possible to align the position of the first line for each ITOP signal as shown in FIG. 12, by synchronizing, by means of the phase matching circuit 1112, the rotational phase of the scanner motor 1102 for effecting the main scanning (sub scanning start signal) with the ITOP signal at each generation of the ITOP signal (sub scanning start signal).
It is thus rendered possible to achieve position alignment even after a number of rotations of the photosensitive member or the intermediate transfer member.
However the positional aberration preventing technology based on the above-described configurations assumes that all the environments of the apparatus are ideal, and such technology is therefore insufficient in practice.
For example, the rotation speed of the photosensitive member shows certain fluctuation for example by a variation in the load or by the backlash of the driving transmission gears. Such fluctuation in the rotation speed results in a variation in the phase difference between the main scanning start signal and the sub scanning start signal, whereby a color aberration is generated in case of employing the aforementioned methods of maintaining the position of the laser scanning line constant on the photosensitive member in the image forming apparatus. Such variation can be suppressed to about 1/5 to 1/6 of a line by minimizing the fluctuation in the load of the motor or by improving the precision of the mechanical drive transmission system.
However, if the phases of the sub scanning start signals for the respective colors to be superposed are positioned across the main scanning start signal, there will result an aberration of a line, though the aberration of each line is in fact a fraction of a line.
FIG. 14 is a timing chart showing the timing of image formation in the conventional image forming apparatus, showing a case where the phases of the sub scanning start signals of the respective colors are positioned across the main scanning start signal.
As shown in FIG. 14, as the sub scanning start signal 1204 for the first rotation is generated slightly before the main scanning start signal (1), the scanning of the first line (1206) is started in synchronization with the main scanning start signal (1), while the scanning of the second line (1207) is started in synchronization with the main scanning start signal (2), and the scanning of the third line (1208) is started in synchronization with the main scanning start signal (3).
However, as the sub scanning start signal 1205 for the second rotation is generated slightly after the main scanning start signal (1), the main scanning start signal (1) cannot be recognized. Consequently the scanning of the first line (1207) is started in synchronization with the main scanning start signal (2), and that of the second line (1208) is started in synchronization with the main scanning start signal (3).
Consequently there results the aberration of a line between the first rotation and the second rotation. The following description refers to FIG. 15.
FIG. 15 is a schematic view showing a situation where the phases of the sub scanning start signals are positioned across the main scanning start signal in the conventional image forming apparatus, wherein elements same as those in FIG. 14 are represented by same numbers.
Referring to FIG. 15, an image bearing member 1201 such as a photosensitive member or an intermediate transfer member will hereinafter be explained as a photosensitive member. An ITOP sensor 1202 is shielded by a sensor flag 1203 in each rotation of the photosensitive member 1201, thereby generating a sub scanning start signal.
The sub scanning start signal 1204 for the first rotation is generated slightly before the main scanning start signal (1), while the sub scanning start signal 1205 for the second rotation is generated slightly after the main scanning start signal (1), and the first line 1206 in the first rotation and the first line 1207 in the second rotation are mutually displaced by a line. Such situation will be explained in more details with reference to FIG. 16.
FIG. 16 is a timing chart showing the timing of image formation in the conventional image forming apparatus, showing the details of the timing chart shown in FIG. 14, wherein elements same as those in FIG. 14 are represented by same numbers.
In the conventional image forming apparatus, after n count of a video clock signal (video CLK) in synchronization with the main scanning start signal, a memory read-out signal is generated during m count of the video CLK signal and the recording data are read from an unrepresented memory in synchronization with the memory read-out signal. The recording data read from the memory are used for laser scanning for each line and thus recorded on the photosensitive member. The sub scanning start signal is generated at a predetermined position in each rotation of the image bearing member, and becomes effective from the main scanning start signal after the sub scanning start signal is shifted from L-level to H-level, thus generating the memory read-out signal.
In the color image forming apparatus in which the images of plural colors are transferred in superposed manner, the latent image formation or the image transfer is repeated plural times. FIG. 16 shows an example of repeating such process twice, wherein the sub scanning start signal for the first rotation is generated slightly before the cycle of the main scanning start signal while that for the second rotation is generated slightly after the cycle of the main scanning start signal.
As shown in FIG. 16, the sub scanning start signal 1204 generated in the first rotation is generated slightly before the main scanning start signal (1), so that the main scanning start signal (1) becomes effective and the timing of the memory read-out signal for the first line of the image is synchronized with the main scanning start signal (1). Consequently the memory read-out signal is generated after n count of the video clock signal from the main scanning start signal (1).
The sub scanning start signal 1206 generated in the second rotation is shifted to the later side because of a fluctuation in the rotation of the image bearing member.
In such case, the sub scanning start signal is generated slightly after the main scanning start signal (1), so that the main scanning start signal (1) is not detected and the memory read-out signal for the first line of the image is synchronized with the main scanning start signal (2). Therefore, the memory readout signal for the second rotation is generated after n count of the video clock signal from the main scanning start signal (2) as shown in FIG. 16.
Consequently, there results an aberration of a line between the memory read-out signal for the first rotation and that for the second rotation. Therefore, in recording the image data read from the memory onto the photosensitive member in successive lines, the first lines which should mutually overlap are mutually displaced, and the first line in the first rotation overlaps with the second line in the second rotation to result in color aberration.
Thus the conventional technologies have been associated with a drawback of generating an aberration of one line or larger in the image recording start positions of the respective colors, because of the fluctuation in the phase difference between the main sub scanning start signal and the main scanning start signal, resulting from a variation in the rotation speed of the photosensitive member etc., caused by a fluctuation in the load or by the backlash in the driving transmission gears.